1. Field of the invention
The invention relates to a totally redundant mass flow meter, and is particularly applicable to the measurement of the flow of a fluid in an electronic control system in a turbomachine.
2. Summary of the Prior Art
In order to measure the mass flow rate of a fluid, it is known to use a flow meter comprising a casing in which there is an axial passage for the flow of a fluid propelled at a constant angular velocity, a rotor which is caused to rotate by the fluid or an electric motor, and a rotationally restricted turbine on which the fluid exerts a torque proportional to the product of the mass flow rate of the fluid and the angular velocity of the fluid. The value of the torque is measured by controlling the position of the turbine by means of a limited movement torque motor and an angular position sensor. Measurement of the current which supplies the torque motor makes it possible to calculate the torque acting on the turbine and to deduce from it the mass flow rate of the fluid. Such a flow meter is described in U.S. Pat. No. 4,056,976.
It is also known to provide a partially redundant flow meter by disposing on the turbine two measurement systems each comprising a torque motor and a position sensor. However, the two measuring systems are not independent and cannot function simultaneously because it is not possible to apply two restoring torques to the turbine at the same time. These two systems can only be used alternately; switching from one system to the other being effected in the event of a breakdown in one of the measuring systems. However, in numerical control where it is necessary to compare two different measurements carried out by two measurement systems for fault probability or detection tests, the two measurements must be carried out simultaneously.